A thin film transistor (TFT) is a kind of field effect transistor (FET). The basic structure of a TFT is a three-terminal element that comprises a gate terminal, a source terminal and a drain terminal, and is an active element that uses a semiconductor thin film that is formed on a substrate as a channel layer in which electrons or holes move, and has a function of switching the current between the source terminal and drain terminal by applying voltage to the gate terminal and controlling the current that flows in the channel layer.
Currently, polycrystalline silicon film or amorphous silicon film is used as the channel layer of a TFT. Particularly, amorphous silicon film can be formed as a uniform film on a large area tenth-generation glass substrate, so is widely used as a channel layer of a TFT for liquid crystal panels. However, the amorphous silicon film has low mobility of electron carriers (carrier mobility) of 1 cm2/Vsec or less, so its application in TFT for high-definition panels is becoming more difficult. In other words, as the definition of liquid crystals increases, there is a need to use semiconductor thin film that has higher carrier mobility than that of 1 cm2/Vsec of the amorphous silicon film for the channel layer.
On the other hand, polycrystalline silicon film has high carrier mobility of about 100 cm2/Vsec, so has sufficient characteristics as channel layer material for a TFT for a high-definition panel. However, in polycrystalline film, the carrier mobility at the crystal boundary is reduced, so there are problems in that the uniformity in the surface of the substrate is poor, and there is variation in the TFT characteristics. Moreover, in the production process for a polycrystalline film, after an amorphous silicon film is formed at a relatively low temperature of 300° C. or less, the film is crystallized by an annealing process. This annealing process is a special process that employs excimer laser annealing or the like, so a high running cost is necessary. In addition, the size of the glass substrate that can be used is limited to the fifth-generation level, so the cost reduction of the polycrystalline film is limited, and thus the product development is also limited.
Due to this, currently, development is actively being performed to obtain a channel layer material that comprises the best characteristics of both amorphous silicon film and polycrystalline silicon film, and that is also low cost. For example, JP 2010-219538 (A) discloses a transparent semi-insulating amorphous thin film that is a transparent amorphous oxide thin film (a-IGZO film) that is formed by a vapor phase film deposition method, and comprises the elements of indium (In), gallium (Ga), zinc (Zn) and oxygen (O), where the structure after crystallization is InGaO3(ZnO)m (m is a natural number less than 6), has a carrier mobility greater than 1 cm2/Vsec, and a carrier density of 1016/cm3 or less without adding impurity ions, and discloses a thin film transistor that uses this transparent semi-insulating amorphous thin film as a channel layer.
However, even though the a-IGZO film that is disclosed in JP 2010-219538 (A) and formed by a vapor phase film deposition method such as a sputtering method or pulse laser vapor deposition method has relatively high carrier mobility in the range of 1 cm2/Vsec to 10 cm2/Vsec, the amorphous oxide thin film is such that oxide deficiencies inherently occur easily, and the behavior of the electrons as carriers is not always stable with respect to external factors such as heat, so there is a problem in that the operation of a device such as a TFT becomes unstable. Furthermore, it has been pointed out that when a negative bias is continuously applied to a TFT element under visible-light irradiation, a phenomenon, which is unique to amorphous film, occurs in that the threshold voltage shifts to the negative side (illumination negative bias degradation phenomenon), and this phenomenon becomes a serious problem in uses such as liquid-crystal displays.
In regard to this, JP 2008-192721 (A) discloses applying an indium oxide film that is doped with tin (Sn), titanium (Ti) or tungsten (W), or an indium oxide film that is doped with tungsten and zinc and/or tin for the channel layer with the object of obtaining a thin film transistor in which elements can be made on a polymeric substrate without requiring a high-temperature process, and that is capable of achieving high performance and high reliability at low cost. According to JP 2008-192721 (A), by applying the amorphous oxide thin film that is obtained with this technology to the channel layer, the TFT element is able to achieve carrier mobility of 5 cm2/Vsec or more.
Moreover, JP 2010-251604 (A) similarly discloses technology in which an indium oxide thin film is formed using an indium oxide sintered compact that is doped with one or two or more of the elements of tin, titanium, tungsten and zinc as a target by a non-heating sputtering film formation method, after which heat treatment is performed for 10 minutes to 120 minutes at 150° C. to 300° C. With this technology, it is possible to obtain a stable indium oxide film with relatively easy control, while at the same time maintain both the characteristics of having high mobility and being amorphous; and by using this indium oxide film as a channel layer, it is possible to obtain a stable TFT element.
However, the indium oxide films that are obtained by the technology disclosed in the literatures above are amorphous films, so problems such as the easy occurrence of oxygen deficiencies, and the films being unstable with respect to external factors such as heat, and furthermore, the problem of the occurrence of an illumination negative bias degradation phenomenon, which is unique to amorphous film, basically cannot be solved. Moreover, when taking into consideration the use of the films as channel layer material for a TFT of a high-definition panel, achieving even higher carrier mobility is desired.